1) Field of Invention
This invention relates to a drive-by-wire (DBW) vehicle engine output control system for setting the quantity of control of an engine in accordance with the degree of depression of an accelerator pedal by a driver and the mode of operation of a vehicle to electrically control the output of the engine on the basis of the control quantity or for controlling the output of the engine without relying upon operation of the accelerator pedal.
2) Description of the Related Art
Constant-speed control systems for making an automotive vehicle run at a constant speed (autocruise systems) have been provided to date. In those installed in automotive vehicles with a throttle valve mechanically connected to an accelerator pedal as an acceleration operating member, treadling of the accelerator pedal by a driver during autocruising is judged as the driver's need for more acceleration than the current level.
In such a case, the driver can directly operate the throttle valve at opening rates greater than the maximum valve drive quantity by a cruise actuator in autocruising.
The DBW system in which no mechanical connection is provided between an accelerator pedal and a throttle valve is, however, accompanied by the inconvenience that acceleration by the accelerator pedal is not feasible unless a brake pedal is treadled to cancel the autocruise mode.
Such a system therefore requires the preposterous operation that, because the throttle valve returns to a fully-closed position by the cancellation of the autocruising and is then driven to open the same, the response becomes slower and the brake pedal has to be treadled once for acceleration.
Further, the speed control system in a conventional constant-speed control system for making an automotive vehicle to run at a constant speed (autocruise system) is constructed, for example, in the following way.
To obtain an intake air volume corresponding to a target vehicle speed, the position of a throttle valve (the position of a control rod of a governor in the case of a diesel engine) is adjustable. Owing to this adjustment, a vehicle speed achieved or realized on the side of an engine-body system is fed back to calculate the deviation .DELTA.V of the actual speed from the target vehicle speed. Feedback control corresponding to the deviation .DELTA.V is then applied to the position of the throttle valve, whereby the target vehicle speed can be realized.
In an automotive vehicle equipped with such a conventional constant-speed control system, a relatively long time is, however, required to return to a target vehicle speed if a substantial deviation from the target vehicle speed takes place upon occurrence of a sudden change in load on a slope or the like.
Where the deviation .DELTA.V from the target vehicle speed is large in the automotive vehicle equipped with the conventional speed control system, the opening rate of the throttle valve is significantly adjusted (in the case of a diesel engine, the position of the control rod of the governor is substantially adjusted) in order to correct or eliminate the deviation .DELTA.V. This may result in the potential problem that the drive torque of the engine would suddenly change to induce a certain acceleration or deceleration shock and the riding comfort would be deteriorated.
To overcome this problem, it may be contemplated of lowering the open/close speed of the throttle valve upon control and hence limiting the change in the drive torque. There is, however, no linear relation between changes in the opening rate of the throttle valve and corresponding changes in the torque of a drive axle. If the open/close speed of the throttle valve is limited under conditions that the torque of the drive axle undergoes the largest change relative to the change in the opening rate of the throttle valve, the response of the speed control becomes slower under other driving conditions so that precise speed control cannot be performed.
It is, therefore, necessary to vary the quantity of control of the open/close speed of the throttle valve in accordance with complex conditions. This, however, results in the need for a complex speed control system.